What do moviegoers look for in their favorite actors? One thing is the recognizability factor, the idea that there’ll be some comfort in kind of knowing, being familiar with, the person up on the screen, even though said actor is playing someone else. But what happens when that doesn’t happen, like when Charlize Theron played serial killer Aileen Wuernos in “Monster,” or Tom Cruise played fictional Hollywood producer Les Grossman in “Tropic Thunder?” What should happen is that audiences forget who they’re watching, and afterward, appreciate whatever it took — from makeup to acting skill — that fooled them.

Such will be the case with Gary Oldman in “Darkest Hour,” as he commits himself, with everything he’s got (and some makeup assistance), to becoming 65-year-old Winston Churchill. From the moment Churchill is introduced in the film, waking up in bed, in a darkened room, his face lit only by the match he holds to his early-morning cigar, there’s no Gary Oldman to be found. His transformation is complete and unwavering. He is Winston Churchill.

This is not a biography of the British statesman. Joe Wright’s film sticks with a peek into a brief, world-changing period in mid-1940, just a few months after England declared war on Germany, when the former soldier-journalist-Parliament member was named Prime Minister, replacing Neville Chamberlain (Ronald Pickup), who had just been forced to resign from the position.

Neither screenwriter Anthony McCarten (“The Theory of Everything) nor Oldman initially makes it clear why Churchill, at the time bearing the title of First Lord of the Admiralty, takes the position and, in fact, seems uncomfortable when the equally uncomfortable King George VI (Ben Mendelsohn) offers it to him. But it’s not long before he throws himself into it.

The script contains a batch of character traits offering some clues about Churchill: That he’s difficult to work for is proven when his new secretary, Elizabeth (Lily James), runs from his office in tears after a berating over her typing skills; he has a very close marriage to Clementine (Kristin Scott Thomas), who has no qualms about telling him, “You’re not as kind as you used to be; he appears to be a bit doddering (but only early on); he likes his cat and loves his cigars; and he displays a great sense of humor. Gathering his family around him to celebrate the Prime Minister post, he lifts a drink and says, “Here’s to not buggering it up.”

But there’s not a lot of fun to be had in the film. It takes place during a tough, tension-filled moment in history. Hitler has gotten to the point of occupying Poland and Norway, and an attack on England is imminent. One reason Chamberlain was removed was due to his proclivity to favor negotiating with the enemy, rather than fighting them off. He lost the trust of the people and the Parliament. But Churchill isn’t going it have it much easier. Despite his past accomplishments, he’s not exactly the most popular guy in town. Some feel that his rousing speeches — about the necessity of victory, about the possibility of the collapse of Western Europe, about fighting to the end — make him nothing more than a warmonger.

Looking back on it now, it’s clear that Churchill’s unflinching determination was a major reason for Hitler’s defeat, and the film tells the story of how he won the public’s support, how he got them to understand his declaration “Conquer we must, and conquer we shall.”

This slow-building but eventually exciting thriller has Oldman holding it together, but the film also boasts an excellent supporting cast — Oldman’s conversations with Scott Thomas and with Mendelsohn are real highlights — outstanding photography in both light and dark scenes, and bits of cinematic magic such as a sequence that shows Churchill giving one of those phenomenal speeches while the cameras keep flashing back to him dictating it and Elizabeth typing it. Yes, it’s a great film in so many ways, but still, in the end, everyone’s going to be talking about Oldman’s performance.

“Darkest Hour” opens on Dec. 8.

— Ed Symkus writes about movies for More Content Now. He can be reached at esymkus@rcn.com.